The present invention relates to an electronic still camera and more particularly to an electronic still camera of a two-stage operation shutter type.
In a conventional electronic still camera, when a camera operator operates a shutter button, a two-stage operation wherein the shutter button is xe2x80x9chalf-depressedxe2x80x9d first, and xe2x80x9cfull-depressedxe2x80x9d after that.
In the electronic still camera of such a two-stage operation shutter type, while the shutter button is in a xe2x80x9chalf-depressedxe2x80x9d state, the operation of automatically determining photographing conditions such as exposure conditions and white balance (hereinafter referred to as preparative photographing) is carried out. In the preparative photographing operation, various photographing conditions such as a focusing an exposure, an angle of view, and others must be set, so that several times of the photographing operation are carried out at a high speed. When the shutter button is xe2x80x9cfull-depressedxe2x80x9d, an actual operation for writing photographic images into the memory (hereinafter the operation is referred to as actual photographing) under the photographing conditions set by the preparative photographing operation is carried out.
Various data obtained in the preparative photographing may be used by a camera operator as reference data for manual-setting of photographing conditions.
Conventionally, in such electronic still cameras, in both of the preparative photographing and the actual photographing, an optical image of a subject is formed on an imaging surface of a solid-state imaging device such as a charge-coupled device in a similar manner. A timing pulse is applied to the solid-state imaging device from a driver such as a CCD driver at a predetermined rate, and thus a image signal photo-electrically transduced by the solid-state imaging device is read out therefrom in accordance with the timing pulse. This image signal is forwarded to a signal processor, and wherein the image signal is subjected to various signal processings according to the timing pulse at the same predetermined rate. That is, in the preparative photographing operation, a signal processing for automatically determining some photographing conditions such as an exposure condition, a white balance etc, are carried out in this signal processing stage. On the other hand, in this photographing operation, the image signal obtained from the optical image formed under the photographing conditions set by the preparative photographing operation in this signal processing stage is processed so as to be put into practical use.
In the electronic still camera of a two-stage operation shutter type, when a camera operator depresses the shutter button at a stroke, at the depressing-down stroke, the time in the preparative photographing state is very short. Nevertheless, as described above, several photographing operations must be carried out in the short time, so that the timing pulse is unavoidably at a comparatively high pulse rate.
Therefore, conventionally, reading out of the image signal at the time of actual photographing operation and various signal processes are so configured as to be carried out according to the timing pulse at the high rate.
Japanese Patent Application Laid-Open 11-146409 discloses an art for reducing the signal read-out rate at a image photographing operation by a image camera lower than the signal read-out rate at a preview operation. However, the art disclosed in this patent application is used to reduce the crosstalk between image signals in a case of insufficient ringing or frequency characteristics of the analog signal system at the image photographing operation and a defect occurs that it adversely affects the control mechanism based on the signal level of automatic white balance because the crosstalk between signals is different between preview and image photographing operation.
In an electronic still camera using a CCD as a solid-state imaging device, conventionally, the reference voltage for changing an output signal of the CCD by a correlation double sampler and a signal voltage generated on it are sampled respectively and the noise component is removed by taking off the difference between them.
However, in such a low-noise art of a sampling system, a noise component in the high frequency band at time random is generated by the output amplifier of the CCD and the noise component is folded on the frequency band effective as an image signal at a sampling operation as a harmful noise. Therefore, for the purpose of removing the noise component in the high frequency band, a low-pass filter having a cut-off frequency about three times of the signal transfer frequency is conventionally used.
However, one fixed image is obtained from the electronic still camera and on such a fixed image, the noise component in the high frequency band at time random generated by the amplifier is also generated in a fixed state, so that a defect occurs that no sufficient noise reduction effect can be obtained.
In a conventional electronic still camera of a two-stage operation shutter type, in relation to the preparative photographic process as described above, reading out of the image signal at the actual photographing operation and various signal processes are carried out at a comparatively high pulse rate. However, when reading out of the image signal and various signal processes are carried out at such a high pulse rate, in the noise reduction art by correlation double sampling, a defect occurs that the noise component in the high frequency band appears on an image in a fixed state and no sufficient noise reduction effect can be obtained.
An object according to the present invention is to solve the conventional problem and to provide an electronic still camera for obtaining an image with a satisfactory SN ratio.
A first aspect of the electronic still camera according to the present invention, to solve the problem, is an electronic camera for determining photographing conditions from output of a solid-state imaging device prior to an actual photographing operation and generating a photographic image from an output signal of the solid-state imaging device based on the photographing conditions, which is characterized by that the electronic camera has a band-width changing means for reducing the signal output rate for photographic signal output lower than the signal output rate for determining the photographing conditions and reducing the signal passing band-width narrower than that for determining the photographing conditions in the image signal path from the solid-state imaging device.
A second aspect of the electronic still camera according to the present invention is characterized by that the electronic camera is additionally so configured as to set the signal passing band-width of the image signal path is proportional to the signal output rate of the solid-state imaging device.
A third aspect of the electronic still camera according to the present invention is characterized by that the signal output rate is automatically changed in accordance with the exposure time in the photographing operation.
Additional objects and advantages according to the present invention will be apparent to persons skilled in the art from a study of the following description and the accompanying drawings, which are hereby incorporated in and constitute a part of this specification.